tcl-tk: New combined formula for Tcl/Tk.

- Version 8.6.0
- Keg-only because installs X11 headers.
- Defaults to include AquaTk but you can:
  - get the X11 based Tk `--with-x`
  - `brew install tcl-tk --without-tk`
tcl-tk: Fix TK_LIBRARY and remove tcl.rb/tk.rb
expect: Optional --with-brewed-tk
diffutils 3.3
Add diffstat formula

Closes #171.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
fetchmail 6.3.25

Closes #169.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
grep: Add "g" prefix to binary by default

Closes #168.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
grep: use opt_prefix to find pcre